In producing semiconductor devices or liquid-crystal display devices, photolithography is used to form circuit patterns on semiconductor wafers or glass substrates.
Photolithography has an exposure step for irradiating a photosensitive resist layer formed on the substrate to transfer the circuit patterns formed on a photomask to the photosensitive resist layer.
In order to expose finely, it is necessary to form fine patterns by the use of photolithography as recent development of enlarging in semiconductor memories and of speeding up in CPU processors.
The limit of the fineness of exposure depends on a spot size of light focused by lenses. A mathematical model 1 (the Rayleigh equation), which is shown below, expresses the spot size.
(Mathematical model 1)φ=k1×(λ/NA) 
The reference codes cited in mathematical model 1 are shown below.                φ: a spot size;        : a wavelength of light;        NA: a numerical aperture of an objective lens;        K1: a proportional constant of an optical system.        
According to mathematical model 1, shortening wavelength λ or enlarging numerical aperture NA reduce spot size .
When a short-wavelength laser as a light source, such as ArF eximer laser (193 nm) or F2 laser (152 nm) is used, it is necessary to vacuumize the optical system including an optical path to pass the light. Such a short-wavelength light may restrict materials of an objective lens.
In Fluorine (F2) laser, for instance, fluorspar (CaF2), which is transparent to the short-wavelength, can be used as an objective lens.
Shortening of wavelength  causes the problems in minimizing spot size φ as described above.
Enlarging of numerical aperture NA decreases a focal point depth according to a mathematical model 2 shown below.
Decreasing of the focal point depth causes a difficulty in forming fine patterns when a surface of exposed substrates is uneven.
(Mathematical model 2)DOF=k2×(λ/NA2) 
The reference codes cited in mathematical model 2 are shown below.                DOF: a focal point depth;        λ: a wavelength of light;        NA: a numerical aperture of an objective lens;        k2: a proportional constant of an optical system.        
For the purpose of enhancing a resolution limit which depends on wavelength λ and numerical aperture NA, a phase-shift mask is used. The phase-shift mask is a mask having a lattice-like pattern on a mask substrate and phase shifters made of halftone films.
When a phase-shift mask is irradiated with light, phase of the light which transmits the phase shifter shifts a 180-degree. Thus, two kinds of lights, one of which transmits the phase shifter and the other of which transmits a portion adjacent to the phase shifter, offset each other in appearance.
Consequently, light contrast is improved and enables exposure with an accuracy of approximately 50 nm by the use of F2 laser.
But, the use of a phase-shift mask requires high cost exposure equipment such as a stepper, and hence is expensive.
Recently, an exposure method using near-field light is paid attention to. A principle of generation of near-field light is explained below with reference to FIG. 3.
Many electric dipoles arise in a number of atoms of a material by irradiating the material to light. Each pole of the electric dipoles vibrates each other. As shown in FIG. 3, two electric flux lines E1 and E2 arise around a single electric dipole. Electric flux line E1 of the two is a closed curved line. The other electric flux line E2 of the two is a closed curved line connecting two poles of the electric dipole.
Flux E1 is propagated and diffracted as propagated light toward remote field.
Hence, propagated light which is usually observed, is caused by flux E1.
Flux E2 is not propagated to a remote field but is localized at the vicinity of the atom as near-field light.
Thus, near-field light cannot be observed usually.
An exposing process of a photosensitive resist layer, by the use of near-field light, is explained below with reference to FIG. 4.
In exposing a photosensitive resist layer by near-field light, a light-shielding film 104 of a photomask 103 contacts with the photosensitive resist layer 102 formed on the surface of a substrate 101. Then, photomask 103 is irradiated with light through mask substrate 105.
Near-field light arises inside an opening portion 106, which is formed in light-shielding firm 106 for the purpose of exposure.
A part of the near-field light is oozed from opening portion 106 to resist layer 102. Thereby, resist layer 102 is exposed.
The photolithography using near-field light as described above is so-called near-field light lithography.
It is necessary to control polarization of light irradiated to photomask 103 according to the shape of opening portion 106 in the near-field light lithography.
When the shape of opening portion 106 is slit-like, for instance, the irradiated light is polarized in the manner that the electric field of the light is parallel with the slit to transfer the shape of the slit to resist 102.
Since near-field light generates regardless of the dimension of opening portion 106, the near-field light lithography makes it possible to expose finely by miniaturizing the dimension of opening portion 106.
But, near-field light K is oozed only a little to resist layer 102 because near-field light K is localized around the atoms as described above.
Hence, it is necessary to shorten the distance between photomask 103 and resist layer 102.
As shown in FIG. 5, a solid layer 107, on which light-shielding film 104 is formed, directly contacts with resist layer 102 formed on photomask 101. Solid layer 107 serves as a spacer.
The art of FIG. 5 is disclosed in the literature “APPLIED PHYSICS LETTERS Vol 81, No7, 12 Aug. 2002 p.1315”.
As shown in FIGS. 6A and 6B, photomask 103 is arranged above the surface of resist layer 102 formed on substrate 101. The space between photomask 103 and layer 102 is vacuumed to enforce the contact strength between photomask 103 and layer 102. The arts of FIGS. 6A and 6B are disclosed in the literature “AAPPS Bulletin Vol. 11, No3 Sep. 2001 p. 10”.
But, direct contact of the photomask with a photosensitive resist layer as shown in FIG. 5, causes a problem that the photomask and the layer rub each other in the contacting. As a result, the photomask and the resist layer are damaged.
In particular, when a photomask is an original plate which is manufactured with precision and is used for a long time, it is desired to prevent the photo mask from being damaged.